Organic EL devices are known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat-panel displays have been contemplated. Representative of earlier organic EL devices are Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, "Double Injection Electroluminescence in Anthracene," RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. Typical organic emitting materials were formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. Naphthalene, anthracene, phenanthrene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terphenyls, quarterphenyls, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene were offered as examples of organic host materials. Anthracene, tetracene, and pentacene were named as examples of activating agents. The organic emitting material was present as a single layer medium having a thickness much above 1 micrometer. Thus, this organic EL medium was highly resistive and the EL device required a relatively high voltage (&gt;100 volts) to operate.
The most recent discoveries in the art of organic EL device construction have resulted in devices having the organic EL medium consisting of extremely thin layers (&lt;1.0 micrometer in combined thickness) separating the anode and cathode. Herein, the organic EL medium is defined as the organic composition between the anode and cathode electrodes. In a basic two-layer EL device structure, one organic layer is specifically chosen to inject and transport holes and the other organic layer is specifically chosen to inject and transport electrons. The interface between the two layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. The extremely thin organic EL medium offers reduced resistance, permitting higher current densities for a given level of electrical bias voltage. Since light emission is directly related to current density through the organic EL medium, the thin layers coupled with increased charge injection and transport efficiencies have allowed acceptable light emission levels (e.g. brightness levels capable of being visually detected in ambient light) to be achieved with low applied voltages in ranges compatible with integrated circuit drivers, such as field effect transistors.
Further improvement in organic EL devices such as color, stability, efficiency and fabrication methods have been disclosed in U.S. Pat. Nos: 4,356,429; 4,539,507; 4,720,432; 4,885,211; 5,151,629; 5,150,006; 5,141,671; 5,073,446; 5,061,569; 5,059,862; 5,059,861; 5,047,687; 4,950,950; 4,769,292, 5,104,740; 5,227,252; 5,256,945; 5,069,975, and 5,122,711; 5,366,811; 5,126,214; 5,142,343; 5,389,444; 5,458,977.
For the production of full-color EL display panel, it is necessary to have efficient red, green and blue (RGB) EL materials with proper chromaticity and sufficient luminance efficiency. The guest-host doped system offers a ready avenue for achieving such an objective, mainly because a single host with optimized transport and luminescent properties may be used together with various guest dopants leading to EL of desirable hue.
A doped EL system based on the principle of guest-host energy transfer to effect the spectral shift from tris-(8-hydroxyquinolinato)aluminum (Alq) to the dopant molecules has been disclosed by Tang et al U.S. Pat. No. 4,769,292!. Alq is a suitable host for red EL emitters since its emission at 530 nm is adequate to sensitize guest EL emission in the red spectral region. The preferred dopants chosen to provide the red emission in this prior art were 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) and the julolidyl derivative DCJ (Structure 1, supra! R=H). These molecules generally have a high photoluminescence (PL) quantum yield (&gt;70% in dilute solution) and the position of the emission maxima can be readily shifted by certain modification of the DCM structure. Furthermore, in both PL and EL, a significant red-shift in emissions was observed with increasing dopant concentration in the Alq host. Thus, efficient red EL emitters with suitable hue can be found among molecules in this DCM class. However, the luminance efficiency of the Alq/DCM system is compromised by two factors. First, the spectral bandwidth of the emission is rather broad. As a result, a suitable red hue can be obtained only with the dominant emission in the deep red region. The broadness of the emission band yields a significant portion of photons in the long wavelength spectral region where the eye is not sensitive resulting in a loss of luminance efficiency. Second, the EL efficiency of the guest-host system is highly dependent on the concentration of the guest in the host matrix. The concentration quenching effect, presumably due to the aggregation of guest molecules, is relatively strong in the Alq/DCM system. A further loss in luminance efficiency would result if a concentrated guest-host system is necessary to provide for an adequate red hue in the EL emission.
Other considerations in the fields of fluorescence and electroluminescence applications are the purity of fluorescent materials and the degree of synthetic complexities, including consideration of yield loss due to post-process purification procedures. In the aforementioned patent, while EL efficiency is adequate for DCM, its emission peaks at around 590 nm. As a result, its emissive color appears orange. The synthesis of DCJ provides a red-shifted fluorescence compared to that of DCM. However, the EL efficiency was reduced due to increased concentration quenching. And, the preparation and subsequent purification of both DCM and DCJ are complicated by the inevitable generation of significant amount of an unwanted corresponding bis-condensed dye caused by the further reaction of the "active" methyl group present in the fluorescent dye molecules. The bis-condensed byproduct of DCM has been identified as 4-(dicyanomethylene)-2,6-bis(p-dimethylaminostyryl)-4H-pyran which absorbs the DCM fluorescence band, thus diminishes or extinguishes its fluorescence (Hammond, Optics Comm., 1989, 29, 331). Furthermore, once the bis-condensed dye is formed in the reaction mixture, it is difficult to remove , particularly in a large scale preparation. Accordingly, it is desirable to provide a fluorescent compound useful in EL applications which has a relatively high EL efficiency, a desired emission in the red region of the spectrum and is easy to synthesize and to purify.
Chen et. al. in a publication titled, "Design and Synthesis of Red Dopants for Electroluminescence" (Proc. 2nd Internat. Sym. Chem. Functional Dyes, 1992, 536) describes the synthesis and EL properties of DCJT (Structure 1, R=CH.sub.3) by introducing steric spacer groups which reduce undesirable concentration quenching, thus providing enhanced EL efficiency in the desired red spectral region. However, the preparation and subsequent purification of DCJT are also complicated by the inevitable generation of significant amount of an unwanted bis-condensed dye (Structure 2 supra!, R=CH.sub.3), which only has a very weak fluorescence in the near infra-red region of the visible spectrum. The contamination of this unwanted byproduct tends to decrease the fluorescence efficiency of DCJT, counteracting the effect of reduction of concentration quenching. When the above mentioned fluorescent compounds are used as dopants in the EL host matrices, such as for example, Alq, the aforementioned undesirable concentration quenching result in correspondingly reduced EL efficiency. ##STR2##